Modification of unsaturated polyester resins for viscosity control

ABSTRACT

An unsaturated polyester resin and low profile additive containing diketo groups.

FIELD OF THE INVENTION

The present invention relates to unsaturated polyester resins and lowprofile additives.

BACKGROUND OF THE INVENTION

Unsaturated polyester (UP) resins are one of the most widely usedthermoset polymers. Their major applications are composite productsmanufactured by compression molding in the form of sheet moldingcompounds (SMC) or bulk molding compounds (BMC), injection molding inthe form of BMC, resin transfer molding (RTM), casting, and hand lay-up.Because of their light weight, high strength and non-corrosive nature,unsaturated polyester resins have replaced sheet metal in manyapplications, particularly in the automotive, electric and homeappliance industries.

Unsaturated polyester resins are typically made by reacting anunsaturated dicarboxylic acid or anhydride, such as maleic anhydride orfumaric acid, with a polyol such as propylene glycol to form a lowmolecular weight unsaturated polyester resin (LMWUPR). For SMC and BMCapplications, such LMWUPR's are generally thickened for easy handlingand good fiber carrying characteristics during mold filling.

Chemically, thickening or "maturation" occurs by linking various LMWUPRmolecules together to form polymer chains of considerably highermolecular weight. Typically, this is done by adding to the system a di-or multi-functional compound which couples two or more polyestermolecules together via their terminal hydroxyl and/or carboxyl groups.Because, the LMWUPR molecules usually contain more than two functionalgroups, the actual product formed is more typically a complex network ofinterconnected polymer chains rather than discrete individual chains.

Compounds used for thickening LMWUPR's are known in the art by variousterms such as "thickness", "thickening agents" and "maturation agents."Essentially two types of compounds are used for this purpose. One typecomprises Group IIA metal oxides and hydroxides. MgO is the most commonagent of this type. It is now well accepted that maturation with thistype of agent occurs via formation of ionic bonds through the reactionof MgO or analogue with the carboxylic acid end groups of the polyestermolecule.

The other type of maturation agent is diisocyanate. Diisocyanatesoperate by forming covalent bonds, specifically urethane linkages, withthe terminal hydroxyl groups of the polyester molecule.

Each type of maturation agent has its own advantages and disadvantages.For example, it is desirable that viscosity increase occur very rapidlyduring maturation and further that viscosity remain stable for extendedperiods of time once maturation is completed. Diisocyanate maturationagents exhibit this desirable property, but MgO-type maturation agentsdo not. Moreover, MgO-type maturation agents are very sensitive tohumidity after maturation, whereas diisocyanates are not.

On the other hand, it is desirable during molding that the UPR resinexhibit good material flow. This facilitates complete filling of themold as well as complete wetting of the filler and other ingredients inthe system by the UPR. The ionic bonds formed when MgO-type maturationagents are used weaken at the elevated temperatures encountered inmolding. This results in reduced compound viscosity and hence thedesired material flow. The covalent bonds formed with isocyanate typethickeners, however, do not weaken at molding temperatures and hencematerial flow is more difficult.

It is well known that molded articles made with conventional UPR resinsoften exhibit poor surface finish. This is believed due to the fact thatUPR's shrink somewhat as a result of the molding operation. To deal withthis problem, it is also well known to add to the system (i.e, the totalcomposition including both the UPR and all other ingredients) certainingredients known as low profile agents (LPA). Although LPA's areeffective, good material flow during molding is still necessary toobtain finishes of the highest quality. The reduced material flowencountered when diisocyanates are used as thickeners reduces LPAeffectiveness in these systems, which in turn may lead to significantfinish problems.

Attempts have been made to develop UPR systems whose viscosity profileexhibits all of the above beneficial properties, namely rapid increaseduring maturation, long term stability and significant viscosityreduction during molding. For example, one proposal has been to use bothMgO type and diisocyanate type thickeners in the same system. See Melby,E. G. and Castro, J. M., 7, Ch. 3 Comprehensive Polymer Science,Pergamon Press, Oxford, UK (1989), the disclosure of which isincorporated herein by reference. To date, however, such systems havenot been found effective as a practical matter.

Accordingly, it is an object of the present invention to provide a newUPR system which exhibits the rapid and stable thickening properties ofa diisocyanate-thickened UPR system but which, at the same time,exhibits the reduction in viscosity and hence good material flow atmolding conditions exhibited by UPR systems thickened with MgO typematuration agents.

In addition, it is another object of the present invention to providenovel components of UPR systems, in particular novel modified lowmolecular weight unsaturated polyesters and novel low profile additives,which can be used either individually or together to form the above UPRsystems.

In addition, it is a still further object of the present invention toprovide a novel technique for formulating specific UPR systems whichallows the desired viscosity profile to be imparted to the system, eventhough the specific components thereof vary widely.

SUMMARY OF THE INVENTION

These and other objects are accomplished by the present invention inaccordance with which diketo functional groups are incorporated into themolecules of the uncured unsaturated polyester resin system.

The diketo functional group has a decomposition temperature essentiallythe same as or only slightly below the temperatures achieved in most UPRmolding operations. Accordingly, when a UPR system incorporating diketogroups is heated during molding, the diketo groups decompose. Thisresults in a severing of at least some of the chemical linkages bondingtogether various parts of the thickened polymer network which, in turn,causes the network to be subdivided into smaller segments. The netresult is that the viscosity of the UPR mass decreases, and hence thedesired amount of material flow is realized. Moreover, this result isrealized even if a diisocyanate is used as the maturation agent, sincean MgO-type maturation agent is no longer relied on to provide thedesired viscosity decrease function.

Thus, in accordance with the present invention diketo groups instead ofMgO type thickeners are relied on to provide the desired property ofviscosity reduction during molding. As a result, diisocyanatethickeners, alone or in combination with MgO-type thickeners, can stillbe used to provide their beneficial properties of rapid viscosityincrease upon maturation and long term viscosity stability. Thus, bothsets of beneficial properties can be provided in the same UPR systemvery easily. Moreover, by varying the manner and amount of diketo groupincorporated into the system, as well as the relative portion ofdiisocyanate versus MgO type maturation agent used in the system, theviscosity profile of the system can be tailored as desired, even ifsignificantly different types of polyol, unsaturated acid/ester, LPA'sand other components are used in a particular application.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is more thoroughly illustrated in the drawingswherein

FIG. 1 is a graph comparing the viscosity profiles during maturation ofa UPR system produced in accordance with the present invention with twoUPR systems of the prior art, one of which uses MgO as the thickener andthe other of which uses methyl diisocyanate as the thickener;

FIG. 2 is a graph illustrating the viscosity profiles of a number ofmodified UPR systems of the present invention during curing;

FIG. 3 is a graph illustrating viscosity of two thickened and oneunthickened UPR system of the present invention as a function of shearrate;

FIG. 4 is a graph illustrating the viscosity profile of an unthickenedUPR system of the present invention, during curing, as a function of anapplied shear stress;

FIG. 5 is a graph illustrating the viscosity profile of an MgO-thickenedUPR system of the present invention, during curing, as a function of anapplied shear stress;

FIG. 6 is a graph illustrating the viscosity profile of adiisocyanate-thickened UPR system of the present invention as a functionof an applied shear stress;

FIG. 7 is a graph illustrating variations in viscosity profile duringcuring made possible in accordance with the present invention bysuitable selection of the thickening agent and the amount of inventivemodified UPR included in the system;

FIG. 8 is a graph similar to FIG. 1 showing viscosity profiles duringthickening of a number of UPR systems including one in which a modifiedLPA in accordance with the present invention is included in the UPRsystem; and

FIG. 9 is a graph illustrating the relationship between viscositychange, resin temperature and heating time in two UPR systems usingconventional LPA's and a third UPR system using a modified LPA producedin accordance with the present invention.

DETAILED DESCRIPTION

In accordance with the present invention, diketo groups are introducedinto the uncured molecules of the UPR system in such a way thatdecomposition of the diketo bond in response to the heat encounteredduring molding severs the long chain high molecular weight polyestermolecules of the UPR into shorter chain lengths. This in turn, causes areduction in system viscosity and hence allows better material flowduring the molding operation.

There are a number of different ways that diketo groups can beintroduced into the molecules of the UPR system in accordance with thepresent invention. For example, the diketo group can be inserteddirectly into the uncured UPR molecule. Alternatively, in systems inwhich an LPA is chemically bonded to the polyester chains via terminalfunctional groups on the LPA, the diketo group can be inserted into theportion or linkage of the LPA molecule bonding to the polyestermolecules. Regardless of which way the diketo group is incorporated intothe non-cured UPR system, the result will be to allow viscosityreduction and hence improvement of material flow and hence shrinkagecontrol during molding, even when diisocyanates are used as thematuration agent.

Diketogulonic Acid

An easy way to incorporate the diketo group into the UPR system inaccordance with the present invention is by the use of diketogulonicacid. This compound can be easily produced by reacting ascorbic acidwith hydrogen peroxide in aqueous solution at room temperature. Thissynthesis is described in Penney, J. R. Zilvz, S. S., Biochem J., 39, 1(1945). The reaction mechanism is illustrated in Reaction (1) below:##STR1##

As can be seen from the above, 2,3-diketogulonic acid contains acarboxyl group on one side of the diketo group and three hydroxyl groupson the other side of the diketo group. With this arrangement offunctional groups on opposite sides of the diketo moiety, the compoundcan be inserted between different units in an otherwise conventional UPRsystem to thereby provide the diketo group as an integral, internallinkage in the UPR molecules.

In accordance with the invention, compounds other than 2,3-diketogulonicacid can also be employed as the source of the diketo groups. So long asthe diketo compound has at least two functional groups, with at leasttwo of these functional groups being separated by the diketo linkage, itcan be used. Of course, compounds which introduce functional groupswhich interfere with the desired chemistry of the other ingredients ofthe system should be avoided. Whether a particular diketo compound canbe used in a particular application can easily be determined by routineexperimentation. In any event, 2,3-diketogulonic acid is preferredbecause it is inexpensive and easily prepared from readily availablereactants.

Modified Unsaturated Polyester Resins

Incorporating a diketo group directly into the polyester molecule of aUPR system using 2,3-diketogulonic acid or analogue can be done in avariety of different ways. Perhaps the easiest way to do this is toattach a unit containing the diketo group in its interior to theterminal carboxyl groups of a conventional LMWUPR as received from themanufacturer. This can easily be accomplished by reacting a conventionalLMWUPR having terminal carboxyl groups with a diamine such as1,6-hexanediamine to couple the diamine to the LMWUPR via an amidelinkage and thereafter reacting the amine group on the other end of thediamine with 2,3-diketogulonic acid.

This synthesis is illustrated below in the following Reactions (2) and(3). ##STR2##

Both of these reactions are simple amide formation reactions and can beeasily accomplished by mixing the compounds together at room temperatureuntil the reaction is completed.

In this synthesis, many other types of diamines can be used instead of1,6-hexanediamine. For example, p-xylylenediamine and 1,4-butyldiaminecan be used. Indeed, basically any other diamine can be used so long asthe functionality described above is not adversely affected. Moreover,dipyridino compounds can also be used. Examples of suitable dipyridinocompounds are 1,2-bis(4pyridyl)ethane, 1-2-bis(2-pyridyl)ethane,1,2-bis(4-pyridyl)butane.

In this regard, it will be readily understood that the purpose of thediamine or dipyridino compound is to enable easy attachment of the2,3-diketogulonic acid group to the terminal carboxyl group of theLMWUPR in such a way that the diketo group is preserved and at the sametime at least one hydroxyl group on the opposite side of the diketogroup is preserved for subsequent reaction with the maturation agent. Inaccordance with the invention, any di- or multi-functional amine orpyridino compound, and indeed any analogue of 2,3-diketogulonic acidincluding those having terminal carboxyl groups in place of or inaddition to the terminal hydroxyl groups on the opposite end of themolecule, can be used so long as this basic functionality is not lost.Determination of which particular multi-functional amine or pyridinocompound and which particular diketo compound should be used in aparticular application depends on the other components in the system aswell as the desired operating properties of the target system. This caneasily be determined through routine experimentation, keeping in mindthe above constraint as to the basic purpose and function of the diamineand the diketo groups.

For example, it has been found that the modified unsaturated polyesters(MUP) produced as described above from 1,6-hexanediamine exhibits acomparatively low compatibility with styrene. This is believed due, atleast in part, to the fact that 1,6-hexanediamine is not very soluble instyrene. The strong hydrogen bonding contributed by the six hydroxylgroups of the MUP is also believed to reduce styrene compatibility. Inaccordance with the invention, styrene compatibility can be improved byusing components and making adjustments which make the resultant MUPmore oleophillic.

For example, aromatic diamines such as p-xylylenediamine can be used toreplace 1,6-hexanediamine in the synthesis since such compounds are moreoleophillic than 1,6-hexanediamine. Furthermore, to reduce the hydrogenbonding effect, some of the terminal hydroxyl groups can be capped withsuitable organic agents. For example, phenyl isocyanate can be used tocap some of these hydroxyl groups. In any event, selection of theparticular MUP to use in a particular application depends on the othercomponents in the system and can easily be determined by routineexperimentation.

The foregoing technique of incorporating a diketo group into the UPRmolecule by amidation with a diamine or analogue followed by amidationwith 2,3-diketogulonic acid is particularly useful for LMWUPR moleculeswhose terminal functionality comprises carboxyl groups. Should aparticular LMWUPR molecule exhibit other types of terminalfunctionality, then other techniques can be used to incorporate thediketo group into the polymer molecule.

For example, if a particular LMWUPR includes terminal amino or pyridinogroups, then the first step of the above synthesis, amidation with adiamine or analogue, can be eliminated and the diketo group incorporatedby reacting the amino- or pyridino-terminated LMWUPR directly with2,3-diketogulonic acid or analogue. In the same way, LMWUPR terminatedwith epoxy functional groups can be reacted directly with2,3-diketogulonic acid or analogue for imparting the diketo group to themolecule. However, in this case the majority of 2,3-diketogulonic acidmoieties would attach to the UPR via the hydroxyl rather than thecarboxyl group of the diketogulonic acid molecule because of thepreferential affinity of epoxy functionality for hydroxyl rather thancarboxyl groups. In a similar way, a diexpoxy compound such as thediglycidyl ether of Bisphenol-A could be used in place of a diamine forattaching a functional group (i.e., an epoxy) to a LMWUPR molecule forsubsequent reaction with 2,3-diketogulonic acid. Such a synthesis wouldbe particularly suitable for UPR's whose major functionality is hydroxylinstead of carboxyl. In any event, there is no restriction in accordancewith the present invention on how the diketo group is incorporated intothe polyester molecule. Any technique can be used which adds a diketogroup to a functional group of the UPR and which also provides anadditional functional group on the other side of the diketo groupcapable of bonding to the thickening agent intended to be used in thematuration process.

Diketo Modified Low Profile Agents

Still another way of introducing a diketo group into the UPR system, inaccordance with the present invention, is to form a modified low profileadditive (LPA-M) containing the desired diketo group.

As mentioned above, some types of LPA's have terminal carboxyl,hydroxyl, epoxy and/or amine groups which react to form ionic bonds withterminal carboxyl or hydroxyl groups in the UPR resin, therebychemically attaching the LPA molecule to the UPR. In accordance withthis aspect of the invention, such an LPA is reacted with a unitcontaining the desired diketo group to thereby form a modified LPA(LPA-M) having a diketo group in the linkage intended to connect the LPAto the polyester molecule of the UPR system.

Producing modified LPA's of this type is most easily done by the sametechnique described above in connection with producing modified LMWUPRmolecules. For example, for carboxyl-terminated LPA's the diketo groupcan be easily incorporated by reacting the LPA with a diamine ordipyridine followed by reaction with 2,3-diketogulonic acid. Thissynthesis is the same as illustrated in the above Reactions (2) and (3),except that the LPA forms the backbone of the molecule reacted insteadof the LMWUPR molecule. Also, in this case, the LPA does not necessarilyrequire multiple carboxyl groups, although as a practical mattermultiple carboxyl will be present. As in the case of Reactions (2) and(3) above, the reactions involved in this synthesis also are simpleamidation reactions which can be easily carried out by mixing thereactants together, with stirring, at room temperature until thereaction is completed.

For LPA's terminated with hydroxyl, amino and epoxy functional groups,the other techniques described above in connection with adding diketofunctionality to LMWUPR's containing the hydroxyl, amino and epoxyfunctionalities can be used.

A particular advantage of the modified LPA's of this aspect of thepresent invention is that they can be used to enhance shrinkage controland hence surface finish of the molded products ultimately produced.Attaching the LPA to the UPR molecule in such a way that this linkagebreaks during molding allows the UPR and LPA to readily form thedissimilar phases that are necessary for good shrinkage control. At thesame time, reliance on diketo groups instead of an MgO-type maturationagent to provide the property of good material flow during moldingallows a diisocyanate to be used as the maturation agent, either aloneor in combination with others, thereby providing the desired rapidviscosity increase and long term stability made possible by thesecompounds. As a result, a UPR system can be produced which not onlyexhibits rapid viscosity increase, long term viscosity stability andgood material flow during molding, but also makes possible theproduction of molded articles having excellent "Class A" finishes, asdesired.

Other System Components

The present invention is applicable to all UPR systems in which lowmolecular weight unsaturated polyester molecules are polymerized intohigher molecular weight polyester chains which, in turn, are thenreacted during molding via their ethylenic unsaturation to formcross-linked matrices. As well appreciated by those skilled in the art,particular components of such systems, in particular, the identities andamounts of the dicarboxylic acid and/or anhydride and the polyol canvary widely. The present invention is applicable to all such systems,there being no restriction on the type or amount of unsaturatedcarboxylic acid and/or anhydride or polyol from the invention.

In the same way, UPR systems used in practice today contain many othercomponents in addition to the unsaturated polyester resin. For example,most such systems contain a significant amount of other ethylenicallyunsaturated comonomer for incorporation into the system by additionpolymerization during the curing reaction. Examples of such comonomersare styrene, methyl methacrylate, dimethyl styrene and vinyl toluene.All such comonomers can also be incorporated into the UPR systemsproduced in accordance with the present invention.

Another conventional ingredient oftentimes included in UPR systems, asdescribed above, are low profile agents (LPA's). As described above, lowprofile additives having terminal carboxyl, amine, epoxy, pyridino orhydroxyl groups can be used to particular advantage in accordance withthe present invention. In addition, however, any other material used asan LPA can also be used for this purpose in the UPR systems of thepresent invention. Examples of such materials are polyethylene,polystyrene, saturated polyesters, polymethyl methacrylate, saturatedpolyester urethanes, styrene butadiene copolymers and so forth.

Other well known components in unsaturated polyester resin systems arecarrier resins, inhibitors, catalysts, mold release agents, pigments andfillers such as fiberglass, chopped glass roving, particulate materialssuch as calcium carbonate, etc., and so forth. All such materials canalso be included in UPR systems produced in accordance with the presentinvention.

Relative Proportion of Diketo Group

There is no real limit on the amount of diketo group that can beincorporated into a particular UPR system in accordance with the presentinvention. Essentially any amount can be used.

In practice, the amount of diketo group to be incorporated into aparticular system depends on many factors including the amounts andidentities of the LMWUPR used, the amount of branching in this LMWUPRand the desired amount of branching in the target thickened UPR system,the identity and amount of the maturation agent, the identity and amountof the other components such as fillers, extenders, etc., the type ofinitiator and the anticipated molding temperature, and mostsignificantly the desired amount of material flow during curing to beexhibited by the product UPR. Once all these considerations are takeninto account, one skilled in this art can easily determine by routineexperimentation the precise amount of diketo group to incorporate into aparticular system as well as to achieve the desired viscosity profile ofthe product UPR.

WORKING EXAMPLES

The following working examples are provided to more thoroughlyillustrate the present invention:

I. UPR SYSTEMS INVOLVING DIKETO-MODIFIED LOW MOLECULAR WEIGHTUNSATURATED POLYESTER RESIN MOLECULES

A series of experiments was conducted in which a modified unsaturatedpolyester resin containing diketo linkages (MUP) was first produced,thereafter this MUP was thickened with various thickening agents, andfinally this thickened MUP was cured. The rheological properties of thesystem were monitored during thickening and also during subsequentcuring.

EXPERIMENTAL

Materials

The unsaturated polyester resin used in these experiments was suppliedby Cook Composites and Polymers under the designation (B95). B95 resincomprises polymerized maleic anhydride and propylene glycol havingcarboxyl groups on both ends. It has a number average molecular weightof 1500 and an average of 9.5 C═C bonds per UP molecule. Styrene (ST)(Aldrich Chemical) was used as received. In experiments in which highermolding temperatures were involved, TBP (t-butyl peroxybenzoate,Atochem) with a molecular weight of 194, an activation energy of 33.0kcal/mole, and a half life at 101° C. of 10 hours was used as theinitiator. At lower temperatures, PDO (t-butyl peroxy-2-ethyl hexanoate,Atochem) with a molecular weight of 216, an activation energy of 34.0kcal/mole, and a half life at 77° C. of 10 hours was used as theinitiator.

Synthesis of Modified Unsaturated Polyester (MUP)

The B95 resin was dissolved in dichloromethane (DCM) to form a 33 wt. %polymer solution. Next, 1,6-hexanediamine was added such that the molarratio of amine groups to carboxyl groups in the polymer solution was2.2. The solution was then stirred continuously for half an hour untilthe solution turned basic (i.e., pH value of about 9). The reactionscheme is shown in Reaction (2) above.

Ascorbic acid was used to synthesize 2,3-diketogulonic acid inaccordance with the reaction scheme illustrated in Reaction (1) above.An aqueous solution of 33% ascorbic acid was reacted with hydrogenperoxide overnight at room temperature to form the 2,3-diketogulonicacid.

To produce the desired modified UP, the aqueous solution of2,3-diketogulonic acid was added to the foregoing amine-ended (B95)polyester solution to a molar ratio of 1 to 1. The composition wasstirred for about 2 hours at room temperature until the solution turnedneutral. The final product was found to be a mixture of modified UP,dichloromethane, and water. The solution was .then placed under vacuumat about 35° C. until all the solvents were evaporated. The driedpolymer was checked by a differential scanning calorimeter (DSC 2910, TAInstruments) and a Fourier transform infrared spectrometer (Model 20 DXspectrometer, Nicolet) to make sure that there was no solvent left.

The modified UP resin based so made was found to have a lowcompatibility with styrene. The maximum styrene concentration in theresin styrene mixture was 43 wt %, i.e., the maximum molar ratio ofstyrene to C=C bonds of this modified UP was 1.5. This is believed dueto the fact that 1,6-hexanediamine is not very soluble in styrene.Strong hydrogen bonding contributed by the six hydroxyl groups of themodified UP is also believed to reduce styrene compatibility. Anothermajor drawback of this modified UP was that the viscosity drop of thethickened resin produced upon heating using this MUP was small. This,again, was due to the presence of a large number of hydroxyl groupswhich tended to form a highly branched polymer when the resin wasthickened with diisocyanates.

In order to solve these problems, additional MUP's were prepared inwhich several changes were made. First, an aromatic diamine,p-xylylenediamine, was used to replace 1,6-hexanediamine. With thisaromatic diamine, the compatibility of the modified UP and styrene wasgreatly improved. A molar ratio of 2 between styrene and C=C bonds ofthe modified UP could be easily achieved. To reduce the hydrogen bondingeffect, phenyl isocyanate was used to cap some of the hydroxyl groups ofthe modified UP resin. A test was done for capping one, two, and fourhydroxyl groups. The capping process was carried out after the modifiedUP was dissolved in styrene. The viscosity of the resin did not changeconsiderably when one hydroxyl group of the modified UP was capped. Whentwo or four hydroxyl groups were capped, the viscosity of the resinreduced greatly. However, capping four hydroxyl groups of the modifiedUP molecule tended to reduce the thickening activity too much, andconsequently, the maturation process with diisocyanates became very long(more than 50 hours). Therefore, capping two hydroxyl groups of themodified UP was chosen for further experimentation. The final version ofthe modified UP (MUP) was dissolved in 50 wt % of styrene to make themolar ratio of styrene to C═C bonds of the MUP equal to two. Cappingsome of the hydroxyl groups also improved the solubility of the MUP instyrene. The foregoing reactions, i.e. formation of the p-xylxylenedianesubstituted intermediate, formation of the diketo modified intermediateand capping, as well as the final chemical structure of this MUP, areillustrated below in the following Reactions (4), (5) and (6): ##STR3##Preparation of Thickened Resin

The above MUP made with p-xylylenediamine, having two hydroxyl groupscapped with phenyl isocyanate and mixed with 50% styrene, as well as theconventional, unmodified polyester, were used to prepare a number ofdifferent thickened UPR systems. In each system, a formulationcomprising either the modified polyester (MUP), the unmodified polyester(UP) or both in styrene was thickened with 1.5 wt % of MgO or 3 to 8 wt% of diphenyl diisocyanate (MDI) at room temperature. The molar ratio ofstyrene to the C═C bonds of the UP resin was always set at 2.0. Also, inorder to inhibit polymerization in those experiments in which curing wasto be avoided, 0.5 wt % of benzoquinone was added to the resin samples.In the other experiments, a polymerization initiator was added which was1 wt % of PDO for 110° C. curing and 1 wt % of TBP for 150° C. curing.

The following Table 1 lists the amounts of the different polyesters andthe amount of styrene in the formulations used to prepare the thickenedsystems.

                  TABLE 1                                                         ______________________________________                                        Composition of the Polyester/Styrene                                          Formulations Used in Making Up Specific UPR Systems                           Sample   UP/MUP     UP,       MUP,  ST,                                       #        weight ratio                                                                             wt %      wt %  wt %                                      ______________________________________                                        1        100% UP    43.2      0.0   56.8                                      2         70% LTP   31.5      13.5  55.0                                                30% MUP                                                             3         60% LTP   27.4      18.3  54.3                                                40% MUP                                                             4        100% MUP   0.0       50.0  50.0                                      ______________________________________                                    

Viscosity Measurement

Viscosity change during maturation as well as upon subsequent heating ofvarious UPR systems made with the above formulations was monitored by aBrookfield viscometer (RVDT, DV-I+ Viscometer, Brookfield EngineeringLaboratories Inc.) with spindle No. 6 and/or No. 7 at 0.5 rpm speed.Each sample was loaded in a glass tube (25 mm in diameter). For heating,the test tube with the thickened resin sample was placed in an oil bathwhose temperature was set at 150° C.

A rheometer developed by Rheometrics, Inc. (a modified RDA II) in thesteady shear mode was used to evaluate the rheological changes of theUPR systems under various shear rates. Viscosity was measured with thesamples being heated from room temperature to 110° C. A pair of serratedaluminum parallel plates, 7.9 mm in diameter, was used as a sample cell.The gap between the two plates was set at 1.1 mm. Experiments wereconducted at different shear rates ranging from 0.01 to 10 sec⁻¹.

Reaction kinetics were measured by the differential scanning calorimeterat the rate of 10° C. per minute. The tests were conducted in volatilealuminum sample pans capable of withstanding at least 2 atmospheresinternal pressure after sealing. The reaction was carried out in thescanning mode from room temperature to 200° C. at a heating rate of 10°C./min.

RESULTS AND DISCUSSION

Characterization of Modified UP

DSC of the above modified UP resin made with p-xylylenediamine andcapped by phenylisocynate showed that an endothermic peak appeared from88° C. to 133° C. This indicates a bond breaking in that temperaturerange. The endothermic peak did not reappear upon a second scan of thesame sample. For further verification, the same experiment was carriedout on the unmodified UP resin and the result showed that there was noendothermic peak. Based on these observations, it is concluded that athermally breakable bond was introduced onto the modified UP resin andthat the decomposition temperature of this bond is about 110° C. Thestructure change of this MUP before and after modification was alsoconfirmed by FT-IR.

It was also observed by DSC that bond breakage temperature was affectedby maturation. In particular, it was observed that the endothermic peakwas broadened and the peak was delayed by adding the thickener. This maybe explained by the increase of molecular weight of the thickened UPsystem.

Maturation Process

The ideal thickening behavior of a UP resin system would be for thesystem to reach saturated viscosity fast and remain stable afterwards.Generally, diisocyanate thickened UP systems exhibit such behavior. FIG.1 shows the comparison of the viscosity profile during maturation of aprior art UP/ST system thickened with MgO or MDI, and the MUP/ST systemof the present invention thickened with MDI. The specific systemscompared in this figure are 100% MUP/ST/3% MDI, 100% UP/ST/8% MDI and100% UP/ST/1.5% of MgO.

As shown in FIG. 1, for the UP/ST/MgO system, the viscosity increasedgradually, taking more than 200 hours to reach 7.0×10⁶ cP. After that,viscosity increased slowly and tended to fluctuate. On the other hand,viscosity increase for the prior art system based on MDI was much fasterand, moreover, resin viscosity remained stable after thickening. This isbecause the urethane linkages formed are stable.

As further shown in FIG. 1, the modified UP resin made in accordancewith the present invention and thickened with MDI resulted in the samethickening behavior as the UP/ST/MDI system. However, the amount of MDIneeded for reaching the same saturated viscosity was different. Toobtain a saturated viscosity of 8.5×10⁶ to 9×10⁶ cP, the unmodifiedUP/ST system required 8 wt % of MDI, i.e., the molar ratio of theisocyanate group to the carboxyl group was 1.15, while only 3 wt % ofMDI was needed for the MUP/ST system, i.e., the molar ratio was 0.25.This difference is due to the introduction of multiple hydroxyl groupsonto the UP molecules which tended to form highly branched polymers andthus less amount of thickener was needed to achieve the same saturatedviscosity.

Viscosity Change During Heating

In order to understand the viscosity changes during heating (i.e curing)of the modified UP resin of the present invention, several thickenedresin systems were compared: UP/ST/MDI, UP/ST/MgO, and three differentMUP/UP/ST/MDI systems. The amount of MDI added was different in thedifferent systems in order to have a similar saturated viscosity. Eachsystem, however, included 1 wt % of benzoquinone as a polymerizationinhibitor.

The compositions of the specific resin systems used in this comparisonas well as the results obtained are set forth in the following Table 2.

                                      TABLE 2                                     __________________________________________________________________________    Comparison of viscosity changes.                                              UPR System                                                                           UP, MUP,                                                                              Thickener,                                                                          μ (cP)                                                                            μ.sub.s (cP)                                                                   μ.sub.m (cP)                                                                    μ.sub.s /μ.sub.m                   Number wt %                                                                              wt %                                                                              wt %  unthickened                                                                          starting                                                                          minimum                                                                            ratio                                    __________________________________________________________________________    #1     0.0 100.0                                                                             3% MDI                                                                              1.3E4  9E6 2.8E4                                                                              321.2                                    #2     60.0                                                                              40.0                                                                              5% MDI                                                                              8.2E3  8.2E6                                                                             5E4  164.0                                    #3     70.0                                                                              30.0                                                                              6% MDI                                                                              7.3E3  9.1E6                                                                             1E5  91.0                                     #4     100.0                                                                             0.0 1.5%                                                                             MgO                                                                              1.3E3  9.5E6                                                                             2.1E5                                                                              45.2                                     #5     100.0                                                                             0.0 8% MDI                                                                              1.3E3  1.5E7                                                                             3.4E6                                                                              4.5                                      __________________________________________________________________________

FIG. 2 shows the viscosity changes occurring during heating of thesesamples measured by the Brookfield viscometer. As seen in this figure,the viscosity of the UP/ST/MDI system, sample 5, was reduced slightlyupon heating from 1.5×10⁷ cP to 3.4×10⁶ cP as the temperature was raisedfrom room temperature to 120° C. The UP/ST/MgO system, sample 4, showeda viscosity drop from 9.5×10⁶ cP to 2.1×105 cP when the temperature wasincreased from 25° to 120° C. This can be explained by the differentbond formation during thickening. The molecules in the MDI based systemwere linked-by covalent bonds which are stable at higher temperatures.In contrast to this, ionic bonds were formed in the MgO based system,which became unstable at elevated temperatures. The viscosity of bothsystems, however, increased after about 900 seconds at which thetemperature was about 110° C. This may be due to the considerablestyrene evaporation and the thermally induced polymerization in theresin system, despite of the presence of benzoquinone.

Among the three MUP based samples of the present invention, viscositydecreased more as the amount of MUP increased. For the 100% MUP/ST/MDIsystem, sample 1, the viscosity decreased from 9×10⁶ cP to 2.8×10⁴ cP asthe temperature increased from room temperature to 120° C. Viscosityshowed a gradual drop in the beginning, then a sharp change around 95°C., which indicates that the diketo groups started to break. When thetemperature was further increased, the lowest viscosity was reached at108° C. After that, viscosity started to increase owing to theevaporation of styrene and thermally induced polymerization of MUP/ST.The starting and the lowest viscosities of the 40%MUP/60%UP/ST/MDIsystem, sample 2, and the 30%MUP/70%UP/ST/MDI system, 'sample 3, were8.2×10⁶ cP to 5×10⁴ cP and 9.1×10⁶ cP to 1×10⁵ cP, respectively.Similarly, a rate change in viscosity drop was found around 95° C. forthese two samples. The lowest viscosities were found at 110° and 111° C.for these two samples.

Theoretically, resin viscosity should drop to the initial value of theunthickened material (see Table 2) if all thickened bonds were brokenupon heating. However, in practice the minimum viscosities of both theUP/ST/MgO and UP/ST/MDI systems were much larger than the initialviscosities of the unthickened resins. This is believed due to the factthat in the UP/ST/MgO system, the ionic bonds between the MgO and the UPmolecules may became weaker at elevated temperatures. However, not allof the bonds broke. Consequently, the minimum viscosity reached duringheating was much higher than the initial viscosity of the unthickenedresin. For the 100%MUP system, the lowest viscosity, 2.8×10⁴ cP wasslightly higher than the unthickened value, 1.3×10⁴ cP. This indicatesthat although most thermally breakable bonds were broken, there werestill some larger molecules formed, perhaps from the urethane linkage ofMDI and the hydroxyl groups of the MUP molecules.

Shear Effect on Gelation of the UP System

During mold filling, the UPR system is under certain shear force. Therelationship between compound viscosity and gelation, on the one hand,and shear rate or shear stress on the other hand, is an important issuesince UP compounds are non-Newtonian fluids which become solids duringmolding. FIG. 3 shows the viscosity changes of an unthickened sample andtwo thickened samples under different shear rates measured by RDA II atroom temperature. The specific systems compared in this figure were 100%MUP/styrene for the unthickened sample and, for the thickened samples,20% MUP/80%UP/styrene with either 1.5 wt % MgO or 6 wt % MDI.

All samples showed the shear thinning phenomenon. For the thickenedsamples using MUP in accordance with the present invention, the MDIthickened system showed less dependence on shear rate than the MgOthickened system. This is probably due to the stronger covalent bondingof the urethane linkage than the ionic bonding of MgO. Therefore, shearforce would result in less influence on the MDI thickened samples thanthe MgO thickened samples.

FIG. 4 shows the viscosity changes during curing of an unthickened UPsystem (i.e. 43.2% UP/56.8%ST) at four different shear rates. Thegelation time was around 160 seconds under the lowest shear rate 0 1sec⁻¹ but increased to 230 seconds when the shear rate was increased to5 sec⁻¹. This implies that the unthickened compounds would have a longerflow time in a manufacturing process with higher shear rates, e.g.,injection molding vs. compression molding.

This shear rate dependence of gelation time corresponds to resultsobtained in previous work. See, Muzumdar, S., Ph.D dissertation, TheOhio State University, 1994. In that work, it was found that thepolymers formed in the reaction of UP resin and styrene had a bimodalmolecular weight distribution. The smaller ones were the primarypolymers formed from the radical polymerization of monomers, oftenreferred to as `microgels`. Because of the multi-functionality of the UPmolecules, the `microgels` had many pendant C═C bonds which made themhighly reactive. The larger polymers observed are believed to be theresult of polymerizations among `microgels`. See, Chiu, Y. Y. and Lee,L. J., submitted to J. Polyp, Sci., Chemistry Edition. Under shear flow,the formation of the larger polymers was delayed. Because resin gelationdepends mostly on the formation of larger polymers, the gelation timebecame longer at higher shear rates.

The relationship between viscosity change upon heating and shear ratewas also investigated for MgO and MDI-thickened modified UP systems ofthe present invention. UPR Systems Nos. 1 and 5 of Table 2 were theparticular systems investigated.

As shown in FIG. 5, the MgO thickened system did not show the samebehavior, as the unthickened system. Instead, the gelation time waslargely independent of shear rate. For the thickened MUP/ST/MDI system,the strong dependence of gelation time on the shear rate was observedagain as shown in FIG. 6. Since the ionic bonds in the MgO thickenedsystem would not totally break upon heating, most molecules in thesystem were connected during polymerization. It is believed thisprevented the formation of the bimodal molecular weight distribution.Consequently, there was little shear effect on gelation time. On theother hand, the thermally breakable bonds broke at elevated temperatureand released the UP molecules in the MDI thickened MUP system.Therefore, the polymer formation in this system was similar to that inthe unthickened UP compound. As a result, the strong shear ratedependence of gelation time was observed in this system.

Compound Viscosity Design

With the different rheological characteristics of UP and MUP thickenedby MgO or MDI as illustrated above, it is possible to design a UPRsystem based on a mixture of these materials to achieve a desiredviscosity profile. An example is shown in FIG. 7. In this figure, thespecific compositions compound are set forth in the following Table 3.

                  TABLE 3                                                         ______________________________________                                        Composition of Samples in FIG. 7.                                             Sample   Composition                                                          ______________________________________                                        a        35% MUP/65% UP/ST/1.5% MgO/1% PDO                                    b        100% UP/ST/1.5% MgO/1% PDO                                           c        100% MUP/ST/3% MDI/1% PDO                                            ______________________________________                                    

For this comparison, viscosity was measured by RDA II under a shear rateof 0.1 sec⁻¹.

As shown in FIG. 7, the 100% UP/MgO system exhibited a significantviscosity drop at the beginning of heating followed within about 30seconds with a significant viscosity increase. This was probably due tothe relaxation of ionic bonds at elevated temperatures in the beginningfollowed by initiation of the curing reaction. On the other hand, forthe 100%MUP/MDI system, resin viscosity decreased gradually at thebeginning of heating, and continued to drop despite of the commencementof the resin cross-linking reaction. This implies that the effect of thebond breakage of the diketo groups was larger than the effect ofcross-linking in this stage. Viscosity started to increase after 140seconds when cross-linking dominated the viscosity change. For the35%MUP/65%UP/MgO system, the viscosity showed a substantial drop at thebeginning of heating, similar to that of the UP/ST/MgO system. Viscosityremained low for about 60 seconds before starting to increase, similarlyto that of the MUP/ST/MDI system. The early viscosity drop was due tothe relaxation of MgO bonds, while viscosity remained low because ofbond breakage of the diketo groups.

This example shows that different viscosity profiles can be built intospecific UPR systems by using different combinations of the resins andthickeners, as desired.

Curing Behavior

The curing behaviors of UP and MUP based systems were investigated bydifferential scanning calorimetry. The DSC scanning data showed atypical bell shaped reaction curve for the unthickened UP/ST system andthe thickened UP/ST/MgO system. The peak temperatures of these twosamples were similar. For the thickened UP/ST/MDI system DSC showed thatthe major reaction occurred at lower temperature. This is believed dueto the catalytic effect of the amide groups in the system. See, Chou, Y.C. and Lee, L. J., "Interpenetrating Polymer Networks", D. Klempner, L.H. Sperling and L. A. Utacki, eds., Advances in Chemistry, Series NO.239, ACS, Washington, D.C. 305(1993). A small secondary reaction peakoccurred at higher temperatures, probably due to the thermally inducedpolymerization.

For the MUP system of the present invention, DSC showed for all cases,whether thickened or unthickened, that an endothermic peak occurredbefore the major reaction peak. A sharp and narrow endothermic peakappeared around 110° C. for the unthickened compound, while a broaderendothermic peak was found around the same temperature for the modifiedUP thickened with MDI. The reaction peak occurred at 148° C. for theunthickened sample, and was accelerated to 135°-136° C. by the presenceof MDI. A shoulder was observed at higher temperatures for the thickenedsamples. This is similar to that of samples based on the regular UPresin. In general, the modified UP resin tended to react slightly slowerthan the unmodified resin. Thickening with MDI tended to shift thereaction peak to a lower temperature and resulted in a secondaryreaction peak (or shoulder).

This confirms that the diketo groups introduced into the UPR system inaccordance with the present invention decomposed immediately beforemajor curing occurs, thereby enabling UPR systems produced in accordancewith the present invention to exhibit excellent material flow during themolding operation.

II. EXPERIMENTS ILLUSTRATING FORMATION OF DIKETO-MODIFIED LOW PROFILEADDITIVES AND THEIR EFFECT ON THICKENING BEHAVIOR AND SHRINKAGE CONTROLOF UPR SYSTEMS

EXPERIMENTAL

Synthesis of Modified Low Profile Additives (LPA-M)

A poly(vinyl acetate) based LPA exhibiting some terminal carboxyl groups(LP40AS, Union Carbide) was used as the standard LPA in this study.LP40AS was also used to synthesize the diketo-containingthermally-breakable LPA's used in these experiments, i.e. the LPA-M.

Diketogulonic acid (DKGA) was grafted onto LP40S via 1,6-hexanediamineas an intermediate using the synthesis of Reactions (2) and (3) above.For this synthesis, 30 wt % LP40AS was dissolved in tetrahydrofuran(THF). Sufficient 1,6-hexanediamine was then added into the polymersolution so that the molar ratio of amine groups to carboxyl groups was2.2. The solution was well stirred and the pH was checked periodicallywith pH paper. After an hour, a diketogulonic acid/water solutionprepared according to Penney and Zilvz as described above was added tothe solution until the polymer solution was neutralized. This mixturewas stirred for 2 hours at room temperature. The modified LP40AS (LPA-M)so formed was precipitated with hexane and dried under vacuum at roomtemperature.

The properties of the LPA-M as formed were characterized using a FT-IRspectrometer (20 DX, Nicolet), and a differential scanning calorimeter(DSC 2910, TA Instruments).

Thickening and Curing

A series of four cured UPR systems were produced to illustrate theeffect of using the LPA-M. In these experiments, a commerciallyavailable UP resin (Q6585, Ashland) with a number average molecularweight of 1580 was used as the UPR. Q6585 consists of polymerized maleicanhydride and propylene glycol and exhibits an average of 10.13 C═Cbonds per molecule. The acid value and the hydroxyl value of this UPresin are the same (about 35). The resin was shipped as a 65 wt %solution of UP in styrene. Extra styrene was added to the UP resin toadjust the molar ratio of styrene to ethylenic unsaturation in thepolyester molecules to a value of 2:1 (about 41 wt % of UP in styrene).In these experiments, both MgO and MDI were used as thickeners. Also, 1wt % t-butyl perbenzoate (TBP) was included in each sample as apolymerization initiator.

The compositions of the four samples are set forth in the followingTable 4. In this table, the numbers given are in weight percents. Theseweight percents for the TBP, MgO and MDI concentrations are based on theweight of the entire system. The weight percents of the styrene andpolyester are based on the total weight of polyester plus styrene only.

                  TABLE 4                                                         ______________________________________                                        Sample UPR System Compositions for LPA Comparisons.                                                         sty-                                            System   LPA    LPA-M    UP   rene TBP  MgO  MDI                              ______________________________________                                        Unthickened                                                                            15.0   --       35.0 50.0 1.0  --   --                               LPA                                                                           LPA/MgO  15.0   --       35.0 50.0 1.0  1.5  --                               LPA/MDI  15.0   --       35.0 50.0 1.0  --   6.0-9.0                          LPA-M/MDI                                                                              --     15.0     35.0 50.0 1.0  --   610-9.0                          ______________________________________                                    

The thickened systems were cured in a metal mold (229 mm×181 mm×6.5 mm)with a cavity size of 30 mm in diameter and 1.5 mm in thickness in alaboratory press (Fred S. Carver). Each sample was cured at 150° for 10minutes under a pressure of 10.3 MPa (1500 psi). To control theviscosity of the thickened resins, heating was started when theviscosity of all thickened resins reached about 2×10⁷ cP.

Material Characterization

a) Viscosity measurement

Viscosity increase during thickening and upon curing were measured usinga Brookfield viscometer (RVTD, DV-I+Viscometer, Brookfield EngineeringLaboratories Inc.) with a No.6 or No.7 spindle at a speed varying from0.3 to 2.5 rpm. Each sample was loaded in a glass tube (25 mm indiameter). For heating, each test tube containing a thickened resinsample was placed in an oil bath whose temperature set at either 100° C.or 150° C. In those experiments where polymerization was undesired, 1 wt% of benzoquinon was added to the resin mixture in place of theinitiator normally present.

b) Morphological observation

The cured samples were cracked open at room temperature, soaked indichloromethane for 3 hours to remove LPA and dried overnight. Thesamples were then sputtered with gold (DESK II, Denton Vacuum Inc.) anda cross-section of each sample was observed with a scanning electronmicroscope (S-510, Hitachi) at 25 kV.

c) Internal surface area measurement

To measure the surface area of the micro-voids in the cured resin, thecracked samples were pre-dried overnight in an oven at 120° C. Thesamples were then placed in a flask and further dried under vacuum for18 hours at 120° C. The internal surface area of samples was measured bythe BET technique with a BET analyzer (Accusorb 2100E, MicroMeritics)with Krypton as an absorbent.

RESULTS AND DISCUSSION

Synthesis of LPA-M

The progress of each reaction was determined by the measurement of thepH of the solution. Because of the presence of carboxyl groups in LPA,the initial pH value of the solution of LP40AS and THF was 3. When1,6-hexanediamine was added to the solution, the pH increased to 9. Nocrosslinked products were formed in the solution. The concentration ofpendant amine groups in the formed polymer was measured by precipitatingthe polymer from solution with n-hexane. The amine concentration wasthen determined by titration with 0.1 N aqueous HCl.

If each carboxyl group of the LP40AS molecule reacted with one of thetwo amine groups of a 1,6-hexanediamine molecule pendant amine group onthe LPA chain, the molar ratio of the pendant amine groups to theinitial carboxyl groups in the LPA should be 1.0. The actual value wasfound to be 1.08. Thus, it was concluded that all carboxyl groups in theLPA had reacted with one amino group of an 1,6-hexanediamine molecule tobond the diamine to the LPA via an amide linking and to present a freependant amine group at the other end of the diamine. These pendant aminegroups were then reacted with the carboxyl groups of the added DKGA.This was done by adding DKGA to the polymer solution until the solutionwas neutralized.

FT-IR spectra confirmed that both 1,6-hexanediamine and DKGA were linkedto the LPA-M.

In order to investigate the thermal decomposition behavior of the diketogroups introduced into the modified LPA, differential scanningcalorimetric (DSC) measurements were conducted. For the unmodifiedLP40AS, no peaks were observed. The modified LPA, however, exhibited aclear endothermic peak above 80 C. This observed peak temperature(113.1° C.) is very close to the thermal decomposition temperature ofDKGA (125° C.). When the LPA-M sample was reheated in the DSC, noendothermic peak appeared. Thus, it is concluded that the modified LPA'sof the present invention incorporate the diketo group and moreover that,upon heating, the diketo groups in the modified LPA irreversiblydecompose staring at 80° C. and reaching a peak decompositiontemperature around 113° C.

Viscosity Changes during Thickening and Heating

FIG. 8 shows viscosity increase during thickening at room temperaturefor three of the UPR resin systems of Table 4. The initial viscosity was1.13×10³ cP for all resins. The viscosity of LPA/MgO system increasedgradually, and reached 10⁶ cP after about 600 hours. After 600 hours,the viscosity still increased slightly. This agreed well with theliterature results. See, Melby E. G., Castro J. M., 7, Ch. 3,Comprehensive Polymer Science, Pergamon Press Oxford UK (1989). After650 hours, the viscosity of the LPA/MgO sample became unstable andvaried between 5×10⁶ and 1×10⁷ cP.

For MgO thickened UP systems, it has been reported that the thickeningbehavior is controlled by two chemical structures. These structures aresensitive to water content and can change easily by varying the humidityin the atmosphere. The unstable viscosity after the thickening of theabove MgO thickened sample is believed due to the chemical structurechange of these thickened bonds. In contrast, the viscosity of the LPAand the LPA-M based resins thickened with 6 wt % of MDI increasedrapidly during the first 40 hours, and then remained nearly unchangedfor 3 months.

Since thickening behavior is controlled by the chemical reaction of thehydroxyl/carboxyl groups of LPA/LPA-M or UP, it is not surprising thatafter the reaction is completed, the viscosity of the MDI-thickenedresins remained unchanged. Moreover, it was found that there was littledifference in thickening behavior between the LPA and the LPA-M basedresins thickened with MDI, except for the final viscosity value. Thefinal viscosity of the LPA-M based resin (1×10⁶ cP) was larger than thatof the LPA based resin (1×10⁵ cP). This is probably due to thedifference of functional groups in LPA-M and LPA. The molarconcentration of the hydroxyl groups in the LPA-M based system wasnearly three times that of the carboxyl groups in the LPA based system.For 6 wt % of MDI, the molar ratio of the isocyanate groups in MDI tothe thickenable functional groups in both LPA and UP (i.e. the carboxylplus the hydroxyl groups) was 0.93 for the LPA based resin and 0.84 forthe LPA-M based resin. Furthermore, each LPA-M molecule has threehydroxyl groups which may result in crosslinking with MDI. Consequently,the final viscosity of the LPA-M based resin was larger than that of theLPA based resin.

In order to investigate the thermal decomposition behavior of thethickened resin in the molding process, the viscosity changes of thethickened resins during curing were measured for the LPA/MgO, LPA/MDIand LPA-M/MDI samples. MDI concentration was set at 9 wt % (i.e. astoichiometric value equal to the thickenable groups in the LPA-M basedresin). To control viscosity differences of the thickened resins,heating of the thickened resins started when each sample's viscosityreached 8×10⁶ cP during thickening.

FIG. 9 shows the relationship between viscosity change, resintemperature and heating time. For the LPA/MDI based system, there is nothermally breakable bond in the thickened resin. As expected, theviscosity of this sample decreased slightly from 8×10⁶ cP to 1×10⁶ cP inresponse to increasing the temperature from 25° to 136° C.

For the LPA/MgO based system, the viscosity started to decrease from avery early stage of heating. The minimum viscosity reached was 3×104 cP.After 20 min, the viscosity of the sample increased drastically probablydue to the thermal polymerization of UP and styrene.

For the LPA-M/MDI based system, the sample viscosity dropped slightlybefore the temperature reached 100° C. Above 100° C., the viscositydecreased rapidly, indicating that the thermally breakable groupsstarted to decompose around 100° C. The minimum viscosity reached was2.2×10⁴ cP. After 15 minutes, the viscosity of the sample increaseddrastically. Compared to the LPA/MgO sample, the rate of viscosity dropof the LPA-M/MDI sample was smaller before 100° C. but larger above 100°C. This indicates that the LPA-M/MDI based resin kept its viscosityduring the early stage of molding when the compound temperature wasbelow 100° C. but reached a lower viscosity than the conventional MgO orMDI thickened compounds when the material temperature was above 100° C.in the mold.

If all thickening bonds were broken upon heating, the viscosity of thethickened resins should reach the initial viscosity of the unthickenedresins (i.e. 1.13×10³ cP). This apparently did not occur. The minimumviscosities of the LPA-M/MDI based system and the LPA/MgO based systemwere both larger than the initial viscosity of the unthickened resins.In the LPA-M/MDI system, MDI probably reacts with both LPA-M and UP. Thechemical bonds between the UP and the MDI molecules would not break byheating, thus, resulting in a larger minimum viscosity. In the LPA/MgObased system, the ionic bonds between the MgO and the LPA/UP moleculesmay become weaker at elevated temperatures. However, all of these bondsprobably do not break. Consequently, the minimum viscosity reachedduring heating was much higher than the initial viscosity of theunthickened resin.

Morphology of Cured Samples

The phase separation mechanism of low shrink UP resins has beenexplained by some researchers as follows. When polymerization starts,the LPA becomes incompatible with the UP resin. This forces UP microgelsto coagulate and separate from the LPA-rich phase. Microvoids form inthe LPA-rich phase or in the interface between the LPA-rich and theUP-rich phases and these microvoids compensate for polymerizationshrinkage of the UP resin.

To determine the effect of using the modified low profile additives ofthe present invention on shrinkage control, the UPR systems describedabove were subjected to curing and the physical appearance of eachsystem was observed both before and after curing. In addition,cross-sections of each cured sample were subjected to scanning electionmicroscopy to produce SEM micrographs, which were also observed andcompared. The following results were observed.

Before curing, all resin mixtures were transparent, except for theLPA/MgO sample. Because of the low solubility of MgO in the resinmixture, the LPA/MgO sample was turbid. After curing, the unthickenedand the LPA-M/MDI samples turned opaque. The cured sample of LPA/MgOhowever, was translucent, while the cured LPA/MDI sample remainedlargely transparent.

If sample opacity is an indication of heterogeneous polymer structureand/or the presence of voids, these results suggested that a strongphase separation and microvoid formation have occurred in theunthickened and LPA-M/MDI samples, some phase separation and microvoidformation occurred in the LPA/MgO sample, while little phase separationand microvoid formation occurred in the LPA/MDI sample during curing.This order is reasonable considering that LPA molecules were not linkedto the UP resin in the unthickened sample and could be easily separatedfrom the thickened UP resin at elevated temperatures in the LPA-M/MDIsystem. Therefore phase separation between LPA and LPA-M and the curedUP resin was strong, and microvoids could be easily formed in thesesamples. In the thickened LPA/MgO sample, the ionic bonds could beweakened but not completely broken by heating. Consequently, phaseseparation and microvoid formation were not as strong as in the firsttwo samples. In the thickened LPA/MDI sample, the LPA and UP moleculesremained chemically bonded during curing. Therefore, little phaseseparation could occur.

Examination of SEM micrographs of the cross-section of the cured samplesrevealed the following information. For the unthickened sample, thestructure of aggregated globules was clearly observed. The averagediameter of these globules was 1.5 micrometers. For the LPA-M/MDIsample, a structure of aggregated globules was also observed. Thediameter of these globules (500 nm) was smaller than those of theunthickened sample. (1.5 um). This is probably due to the change ofcompatibility between UP and LPA when LPA was modified. For the LPA/MgOsample, the globular type structure could be seen. However, manyglobules were linked together. Thus, it is concluded that theperformance of LPA on phase separation was reduced by thickening withMgO. For the LPA/MDI sample, although there were some large globules, noclear phase separation could be observed. Most globules were linked tothe continuous phase because the bonds between the LPA molecules and theUP resin could not be broken.

To quantify the amount of microvoids in the cured samples, surface areasmeasurement by the BET (Brunauer, Emmitt and Teller) technique was usedon the three samples illustrated in FIG. 9 plus an unthickened samplemade from 15 wt % LPA and 50 wt % styrene. The results obtained are setforth in the following Table 5.

                  TABLE 5                                                         ______________________________________                                        Surface Area Measurements                                                     Sample        Maximum Surface Area M.sup.2 g                                  ______________________________________                                        LPA/MDI       0.1-0.2 (est)*                                                  Unthickened LPA                                                                             0.797                                                           LPA/MgO       0.349                                                           LPA-M/MDI     0.674                                                           ______________________________________                                         *Estimated value. Surface area too small to be measured by equipment          employed.                                                                

As shown in Table 5, the surface area of the unthickened LPA sample wasthe highest of all samples. The clear phase separation observed by SEMfor the unthickened sample supports this result. The LPA/MDI sampleshowed the minimum surface area due to poor phase separation. For theLPA/MgO sample, the surface area of 0.349 m² /g was between that of theunthickened sample and that of the LPA/MDI sample. This again indicatesthat the effect of LPA on the shrinkage control was reduced by the MgOthickening.

Of the three thickened samples, the LPA-M/MDI sample showed the largestsurface area (0.674 m² g⁻¹). This value is very close to that of theunthickened sample, which suggests that LPA-M may act as a bettershrinkage controller than the LPA thickened with MgO.

Effect of Temperatures on Sample Morphology

In order to investigate if temperature has any effect on samplemorphology, additional curing experiments were carried out at 125° C.instead of 150° C., again for 10 minutes curing time. The compositionsof the samples remained the same as in Table 5 except that 1 wt % oft-butyl peroctate (PDO) was used as the initiator in order to keep thehigh reaction rate.

During curing, the unthickened sample turned from a transparentyellowish solution to a white opaque solid as in the case of 150° C.curing. Moreover, an SEM micrograph of the cured product showed clearphase separation with aggregation of globules. The average diameter ofglobules was 1.2 micrometers, which is nearly the same as those of thesample cured at 150° C.

The thickened LPA/MgO sample turned from a slightly yellowish turbidmixture to a translucent white solid during curing. The LPA/MgO samplecured at 125° C. was much more translucent than that cured at 150° C. AnSEM micrograph of the LPA/MgO sample cured at 125° C. showed a flakelike structure. This indicates that phase separation in this sample wasless than that cured at 150° C. (FIG. 8c). This, in turn, implies thatthe shrinkage control performance of the LPA thickened with MgO maydecrease when the curing temperature is reduced from 150° C.

The thickened sample of LPA-M/MDI turned from a transparent brownmixture to a slightly brown opaque solid during curing, which wassimilar to the case of 150° C. curing. Moreover, an SEM micrograph ofthe LPA-M/MDI sample cured at 125° C. showed an aggregation structure ofglobules with 500 nm diameters, very similar to that of sample cured at150° C. These results suggest that the efficiency of LPA-M should remainthe same when the curing temperature is reduced from 150° C. to 125° C.

To further investigate the effect of temperature on the morphology ofLPA-M/MDI sample, additional curing experiments were carried out at 80°C. and 60° C. with 2,5-dimethyl-2,5-di(2ethylhexanoylperoxy) hexane(Lupersol 256) as the initiator. At 80° C., the cured sample also turnedto an opaque solid. The sample cured at 60° C., however, was mostlytranslucent. SEM micrographics of the two samples showed a clear phaseseparation can be seen in the 80° C. cured sample, but not in the 60° C.cured sample. This indicates that most thickening bonds did not break at60° C.

Because of reaction exotherm, sample temperature could be much higherthan the mold temperature during curing. To verify this, the temperatureat the center of the sample was measured by a thermocouple duringcuring. For the sample cured at 80° C., the inside temperature reached103° C., while the maximum temperature for the sample cured at 60° C.was only 64° C. These results together with the DSC results mentionedabove confirm that decomposition of thickened bonds in the LPAM/MDIsample determined the morphology of the cured resin and the bonddecomposition could be controlled by changing temperature.

From the foregoing, it can be seen that the modified low profileadditives (M-LPA) of the present invention allow formation of UPRsystems exhibiting excellent material flow during molding, even if suchsystems employ diisocyanates as the maturation agent. Hence, it ispossible in accordance with the present invention to develop specificUPR systems which not only are especially useful in both bulk moldingand sheet molding operations but which also exhibit viscosity profilesmaking them very easy to use.

GENERAL

Although only a few embodiments of the invention have been describedabove, it should be appreciated that many modifications can be madewithout departing from the spirit and the scope of the invention.

For example, it should be appreciated that in actual practice, branchingoften occurs in many different UPR systems. This occurs because one ormore of the polymerizable components in the system has multiple terminalhydroxyl, carboxyl or other condensation-polymerizable groups. Thepresent invention is applicable to all such systems, regardless of theamount of branching employed. Moreover, additional branching can bepurposefully introduced into such systems in accordance with the presentinvention by using multi-functional amines rather than di-functionalamines for incorporating the diketo group into the polyester moleculeand by using LPA's with multi-functional groups.

Furthermore, although the present invention has referred to only twotypes of maturation agents, namely MgO-type compounds and diisocyanates,it should be appreciated that other saturation agents can also be used.For example, certain diepoxy compounds are known to function asmaturation agents in a similar manner to diisocyanates. So long as acompound is capable of coupling multiple LMWUPR molecules together viatheir terminal carboxyl or hydroxyl groups without significantdecomposition of the ethylenic unsaturation therein, it can be used as amaturation agent in accordance with the present invention. Of course,the types and amounts of the other ingredients in the system, includingthe diketo group, will need to be adjusted to accommodate the particularproperties of this material.

Furthermore, although the specific mechanism for incorporating thediketo group into the UPR polymer system in the foregoing description isbased on amidation of the LMWUPR or LPA with a diamine followed byfurther amidation with 2,3-diketogulonic acid, it should be appreciatedthat any technique for introducing the diketo group into the UPR systemcan be employed for this purpose so long as the amount of diketo groupsintroduced into the UPR system and the location of such groups when sointroduced are sufficient to allow viscosity decrease of the systemunder curing conditions as a result of polymer chain severing.

Also, as used herein "low" molecular weight and "high" molecular weightare not intended to refer to any particular molecular weight but ratherto the relation between the molecular weights of the unthickened UPstarting resin and the thickened UPR. Typically, LMWUPR's availablecommercially contain about 4 to 50, more normally about 6 to 20repeating polyol/acid (or anhydride) polyester units and exhibitmolecular weights has low as 300 to as high as 50,000, more normallyabout 1,000 to 10,000. When thickened, these materials typically are inthe form of complex polymer networks, which really have no true"molecular weight" as such. In any event, "low molecular weight" as usedherein is intended to included all UP polymers including oligomers whichcan be thickened to moldable UPR systems. In the same way, "highmolecular weight" is intended to include all moldable UPR systems madewith any such thickened low molecular weight UP polymer.

In addition, it should also be appreciated that, while the foregoingdescription discusses improvement in shrinkage control only inconnection with using the novel modified low profile additives of thepresent invention, improvement in shrinkage control also occurs in theother embarkments of the invention as well, whether or not a modifiedLPA of the present invention is also used.

All such modifications are intended to be included within the scope ofthe present invention, which is to be limited only by the followingclaims:

We claim:
 1. An unsaturated polyester containing diketo groups.
 2. Theunsaturated polyester of claim 1 wherein said polyester is a resincontaining sufficient diketo groups to enable the viscosity of saidresin to decrease when said resin is heated to a temperature of about88° C.-133° C.
 3. The unsaturated polyester resin of claim 2 whereinsaid temperature is about 110° C.
 4. The unsaturated polyester resin ofclaim 2 wherein said diketo groups are present as in integral part ofthe polyester molecules of said resin.
 5. The unsaturated polyesterresin of claim 4 wherein said resin is composed of repeating units oflow molecular weight unsaturated polyester moieties arranged in polymerchains, adjacent pairs of said moieties in said chains being connectedtogether by maturation agents copolymerized into said chains, saiddiketo groups being arranged between adjacent polyester moieties andmaturation agent moieties in said chains.
 6. The unsaturated polyesterresin of claim 5 wherein said maturation agent is an oxide or hydroxideof a Group IIA metal.
 7. The unsaturated polyester resin of claim 5wherein said maturation agent is a diisocyanate or diepoxy compound. 8.The unsaturated polyester resin of claim 5 wherein said unsaturatedpolyester resin is the reaction product produced by(a) reacting a lowmolecular weight unsaturated polyester resin with a diamine ordipyridino compound to produce a first modified polyester product, (b)reacting said first modified polyester product with a multi-functionaldiketo compound, at least two of the functional groups of said diketocompound being separated by the diketo linkage thereof to form a secondmodified polyester product, and (c) reacting said second modifiedpolyester product with a maturation agent to produce a thickenedunsaturated polyester resin composition.
 9. The unsaturated polyesterresin of claim 8 wherein said diketo compound is 2,3-diketogulonic acid.10. The unsaturated polyester resin of claim 2 wherein polyestermolecules of said resin are chemically attached to molecules of a lowprofile additive via linkages containing a diketo group.
 11. Theunsaturated polyester resin of claim 10 wherein said linkages arechemically attached to carboxyl groups of said low profile additive. 12.The unsaturated polyester resin of claim 11 wherein said low profileadditive is polyvinylacetate.
 13. The unsaturated polyester resin ofclaim 10 wherein the linkages attaching said polyester molecules to saidmolecules of a low profile additive are produced by(a) reacting a lowprofile additive having terminal carboxyl groups with a diamine toproduce a first modified-LPA product, (b) reacting said first modifiedLPA product with a multi-functional diketo compound, at least two of thefunctional groups of said diketo compound being separated by the diketolinkage thereof, to form a second modified-LPA product, and (c) reactingsaid second modified-LPA product and molecules of a low molecular weightunsaturated polyester resin together to produce a thickened unsaturatedpolyester resin composition.
 14. The unsaturated polyester resin ofclaim 13, wherein said second modified-LPA product and said lowmolecular weight unsaturated polyester resin are reacted together in thepresence of a diisocyanate to form said unsaturated polyester resincomposition.
 15. The unsaturated polyester resin of claim 13 whereinsaid diketo compound is 2,3-diketogulonic acid.
 16. A low molecularweight modified polyester comprising polymer chains of repeatingpolyester units, said polymer chains having at least two functional endgroups that react with a maturation agent for forming a thickenedpolyester resin system, said functional end groups being separated fromthe remainder of the polymer chain to which they are attached bylinkages, each linkage containing a diketo group attached on one sidethereof to said end group and on the other side to said remainder of thepolymer chain.
 17. The modified low molecular weight unsaturatedpolyester of claims 16, wherein said low molecular weight unsaturatedpolyester comprises the reaction product produced by(a) reacting a lowmolecular weight unsaturated polyester having terminal carboxyl groupswith an organic diamine or pyridino compound to produce a firstpolyester reaction product having terminal amino groups, and (b)reacting said first polyester reaction product with a multi-functionaldiketo compound, at least two of the functional groups of said diketocompound being separated by the diketo linkage thereof, to form saidmodified low molecular weight unsaturated polyester.
 18. The modifiedlow molecular weight unsaturated polyester of claim 17, wherein saiddiketo compound is 2,3-diketogulonic acid.
 19. A modified low profileadditive for use in unsaturated polyester systems, said low profileadditive comprising a solid polymeric material, the polymer chains ofsaid polymeric material having functional end groups that react with amaturation agent for chemically bonding said polymeric material to thepolyester chains of Said unsaturated polyester system, said functionalend groups being separated from the remainder of the polymer chains towhich they are attached by linkages, each linkage containing a diketogroup attached on one side thereof to said end group and on the otherside to said remainder of the polymer chain.
 20. The modified lowprofile additive of claim 19, wherein said modified low profile additiveis the reaction product produced by(a) reacting a low profile additivehaving terminal carboxyl groups with an organic diamine or dipyridinocompound to produce a first LPA reaction product bearing terminal aminoor pyridino groups, and (b) reacting said first LPA reaction productwith a multi-functional diketo compound, at least two of the functionalgroups of said diketo compound being separated by the diketo linkagethereof, to form said modified low profile additive.
 21. The modifiedlow profile additive of claim 20 wherein said diketo compound is2,3-diketogulonic acid.
 22. An unsaturated polyester resin systemcomprising the unsaturated polyester resin of claim 2, a filler and apolymerization initiator.
 23. The unsaturated polyester resin system ofclaim 22 further comprising a ethylenically unsaturated comonomer. 24.The unsaturated polyester resin system of claim 23 wherein saidcomonomer is styrene.
 25. A cured unsaturated polyester resin systemcomprising the product obtained by heating the unsaturated polyesterresin system of claim 23 to cross-link the polyester molecules therein.